This invention relates to procedures and apparatus for electrolytic production of metals in a molten bath, wherein a molten metal lighter than the bath is released at cathode surfaces in the bath while a gas that must be kept from contact with the released metal is generated at anode surfaces in the bath.
In an important specific aspect, the invention is directed to the production of magnesium metal by electrolysis of a magnesium halide, such as magnesium chloride, to which particular reference will be made herein for purposes of illustration. Examples of structures and methods employed for such production of magnesium are disclosed in U.S. Pat. No. 2,785,121, issued March 12, 1957 on an application of A. H. Johnston, and U.S. Pat. No. 3,396,094, issued Aug. 6, 1968 on an application of O. G. Sivilotti et al.
As described in these patents, metallic magnesium may be produced by passing direct electric current between anodes and cathode suspended in facing spaced relation in a molten salt bath, containing magnesium chloride, within an enclosed cell chamber. The current heats the bath to maintain it at a temperature at least above the melting point of magnesium, and effects electrolysis of magnesium chloride in the bath, causing molten magnesium metal to be released at the cathode surfaces while chlorine gas is generated at the anode surfaces. The metal, being lighter than the bath, rises along the cathode surfaces, while the gas rises through the bath in a plume of bubbles from each anode surface to collect in a gas space within the chamber above the level of the bath. Extending above each cathode, but beneath the surface of the bath, is an inverted trough for receiving the rising metal and conducting it to a suitable collection locality external to the main cell chamber. Throughout this process, it is important that the released metal be kept isolated from the evolving gas, to prevent recombination of magnesium and chlorine that would decrease the production of metal, i.e. by returning metal to the bath as magnesium chloride, with resultant impairment of efficiency of the operation. The described procedure may be effectively continuous, e.g. with periodic replenishment of the magnesium chloride content of the bath, removal of product metal from the collecting locality, and withdrawal of chlorine gas from the chamber.
Typically the anodes are fabricated of graphite, while the cathodes may be steel plates. In some instances, the bath employed is reactive with carbon of the graphite anodes (chiefly owing, as presently believed, to the presence of water of crystallization with the magnesium chloride in the bath), and causes progressive consumption of the anodes with resultant tapering of the anode lower ends, although the anodes are formed with vertical sides and remain verticalsided above the locality of this incidental tapering. It has heretofore been proposed, in such case, to orient the cathodes obliquely for the purpose of achieving parallelism with the inherent taper of the lower end portions of the anodes undergoing consumption. It is, however, widely preferred to use a bath (e.g. an anhydrous salt mixture) that is essentially free of substances that react with the anode carbon, in order to avoid the expense and inconvenience of the progressive downward feeding or frequent replenishment of the anodes that is necessary if the anodes are being progressively consumed. When a nonreactive bath is employed, the spaced, facing major surfaces of the anodes and cathodes are conventionally oriented vertically and in parallel relation to each other, throughout the vertical extent of the cell.
The spacing between the facing cathode and anode surfaces, in such a cell, is determined by the requirement that the released metal flowing upwardly along the cathode surfaces be kept away from contact (and consequent recombination) with the plume of gas liberated at the anode surfaces. That is to say, it is necessary that the surface of a cathode be sufficiently far from the facing surface of an anode so that, at every portion of the cathode surface over which the rising metal flows, the metal is safely isolated from the anode gas plume. Thus, in practice, the facing electrode surfaces must be spaced relatively far apart.